This diff supports entering kdb early in the boot when the RB_KDB flag is set on arm platforms.
While trying to get this to work, I discovered that machdep_boot.c doesn’t include opt_ddb.h and so the kernel symbols weren’t getting loaded into the debugger early in the boot (see the call to db_fetch_ksymtab() in freebsd_parse_boot_param()). I have never noticed any problems with symbols in kdb before. I wonder how those symbols eventually get loaded and wonder if now I'm calling it twice.